This invention relates generally to mesa geometry semiconductor light emitters (e.g., lasers, spontaneous emission sources) and, more particularly, to intersubband (ISB) ridge waveguide (RWG) light emitters.
The class of semiconductor light emitters includes a subset known as mesa geometry edge emitters, which in turn includes a particular design known as RWG light emitters. RWG devices typically include an active region and upper and lower cladding regions that bound the active region and form a waveguide. The term ridge waveguide takes its name from an elongated mesa that extends longitudinally along the light propagation axis (e.g., the resonator axis of a laser) and vertically (in cross-section) through at least the upper cladding region and typically also the active region. The mesa provides both lateral current and optical confinement. When suitable pumping current is applied to the active region, light is emitted from either or both ends of the mesa depending on how (or whether) those ends are coated. The current is generated by a voltage applied across a pair of opposing electrodes, one a broad area contact formed on the bottom of the device (e.g., on the substrate) and the other a stripe geometry contact formed on the top of the mesa. The latter is illustratively defined by first depositing a dielectric layer (e.g., SiNx or SiO2) over the mesa, patterning the dielectric to expose the top of the mesa, and then depositing a metal layer over the dielectric layer and the exposed top of the mesa. In this configuration the metal layer typically extends along the sidewalls of the mesa and down to the level of the active region. While the metalization along the side walls and the bottom of the mesa is generally not needed for the basic functionality of the device (only the stripe geometry contact on the top of the mesa is needed), in practice the metal is frequently necessary in order to provide enough area to apply external wiring to the top contact and to provide sufficient conductivity to safely guide the current to the contact area on top of the mesa.
Of course, RWG devices may be used as lasers or as spontaneous emission sources (e.g., LEDs).
The performance of RWG light emitters is a function of many parameters including especially the wavelength of the emission, the temperature of operation, the applied voltage, the width of the mesa, the effective refractive index of the waveguide, the refractive index and the dielectric strength of the dielectric layer, and the losses introduced by the top metal electrode and by the dielectric layer.
Take, for example, the case of ISB lasers, in particular quantum cascade (QC) lasers, that operate at center wavelengths in the mid-IR range of about 3-19 xcexcM These lasers tend to be relatively high voltage devices (e.g., 6-10 V) with higher voltages being required for devices that have a larger number of cascaded stages or that operate at shorter wavelengths.
In QC lasers the mesa geometry provides lateral current confinement, but the exposed sidewalls of the mesa are typically covered with a dielectric layer (e.g., CVD-deposited SiNx or SiO2), as discussed above, that is used not only to define the stripe geometry electrode but also to improve heat removal from the mesa (as compared to a mesa that interfaces with air or a vacuum). This configuration presents two problems: (1) the metal electrode introduces optical loss, which increases the threshold current density of the laser, and (2) the dielectric layer is subject to breakdown, especially under high voltage/high power operating conditions. Conceptually, the dielectric layer could be made thicker to increase its breakdown voltage, and thereby allow for higher voltage/power operation, but in practice thicker dielectric layers are not feasible due to the intrinsic stress/strain of CVD-deposited SiNx or SiO2. Moreover, even though a thicker oxide/nitride would reduce the penetration of the optical field into the overlying metal electrode, it still would be inadvisable in the wavelength range of about 8.5-10.5 xcexcm where the absorption of these dielectrics increases significantly.
Alternatively, higher power can be realized by increasing the number of cascaded stages of an ISB laser, but this approach comes at the price of increased applied voltage, which, of course, raises again the first problem of dielectric breakdown. Another alternative would be to exploit the higher dielectric strength of a polymer as a substitute for either SiNx or SiO2, but in the mid-IR range most polymers (e.g., polyimide) exhibit strong absorption and hence increased optical loss.
Thus, a need remains in the ISB laser art for a dielectric material that has low absorption in the mid-IR range and high dielectric strength to enable high power/high temperature operation.
As noted above, the top metal electrode introduces optical loss in the RWG in the lateral direction, which we have found induces self-mode-locking (self-pulsation) via a self-focusing process. Self-mode-locking can be disadvantageous, particularly if a continuous wave (CW) output or an induced, controlled pulsation is desired. This problem is even more severe in higher power operation and in narrower mesa RWG configurations.
Thus, a need remains in the art of ISB lasers for a waveguide coating that does not introduce significant loss or significant nonlinear refractive index into the waveguiding process.
In order to increase the maximum temperature at which CW operation is possible in ISB lasers, the mesa should be relatively narrow (to insure relatively large surface-to-volume ratio) and the mesa coatings should provide efficient lateral heat conduction through the mesa sidewalls. However, if the coating either introduces significant optical loss, or fails to confine the optical mode strongly, or provides inadequate heat conduction, then the advantages of a narrow ridge may be dissipated.
Therefore, a need remains in the art of ISB lasers for a low loss mesa dielectric coating that provides lateral optical confinement without increasing the threshold current density for lasing and provides efficient heat conduction through the sidewalls of the mesa.
Lastly, in distributed feedback (DFB) versions of ISB lasers, once the grating is formed fine-tuning of the Bragg wavelength (single-mode emission wavelength) can be made only via the effective refractive index of the material penetrated by the optical mode. Temperature affects this refractive index, but once the device temperature is set by the application or other device-related factors, the effective refractive index can be further changed only by altering the refractive index of the sidewall coating, but then only if the mesa is relatively narrow (e.g., a few micrometers). However, as discussed above, use of a narrow mesa has been hindered by the increased optical loss in the top metal electrode.
Thus, a need also remains in the art of DFB ISB lasers for the ability to fine-tune the center wavelength via a low loss coating on a relatively narrow mesa.
In accordance with one aspect of our invention, a mesa geometry semiconductor light emitter includes on the sidewalls a dielectric coating that comprises a chalcogenide (CG) glass.
We have found that these materials have very low loss in the mid-IR range and low refractive index that enhances optical confinement. They are also electrostatically strong and can be deposited, without significant accumulation of strain, as relatively thick layers.
In addition, CG glasses are compatible with current technology used in the fabrication of ISB light emitters.
In an illustrative embodiment of our invention, 12-stage, GaInAs/AlInAs QC lasers operating at about 8 xcexcm have been coated with Ge0.25Se0.75 glass and have exhibited a 30-40 K increase in the maximum CW operating temperature from about 130 K to about 170 K.
In a preferred embodiment, the CG glass coating is sufficiently thick to prevent any significant penetration of optical radiation into the metal electrode that overlays the coating and to provide significant waveguiding of the radiation
In an alternative embodiment of our invention, mesa geometry light emitters include a multi-layered coating that comprises a relatively thin, high refractive index first layer on the sidewalls of the mesa and relatively thicker, lower refractive index second CG layer on the first layer so as to improve the confinement of the optical mode to the mesa.
In another embodiment of our invention, mesa geometry light emitters include a multi-layered coating that comprises alternating CG layers of different refractive index that form a Bragg reflector or an equivalent (such as a chirped grating) on the sidewalls.
In yet another embodiment of mesa geometry light emitters of our invention, the width of the mesa is modulated along the longitudinal axis so as to form Bragg gratings in the sidewalls to increase the surface-to-volume ratio and possibly enhance feedback for the fundamental longitudinal mode. CG coatings are then deposited over the gratings.